Hi Marco,
I am really sorry for the delay of this reply as it is a weekend for us here.
Thanks for your screenshot, it seems like your mini registration custom field is not set completely.
if you would like to use mini registration, you need to go to your backend > Easysocial > profile type > select your profile type > custom field tab > click on the email address field > click on edit tab > click view tab > Appear in Mini Registration - YES
You also need to do the same thing with the username custom field, because this 3 custom field is Mandatory fields e.g username/fullname/email
Hope this help.